The common 12V battery used in many applications is typically of the lead-acid type. Despite having low energy-to-weight ratios and low energy-to-volume ratios, lead-acid batteries are often able to supply high surge currents, meaning they have relatively large power-to-weight ratios. Such characteristics, along with being low cost, make them and other lead-acid type batteries suitable for wide-spread use.
Components, including those used for batteries, are often standardized for mass production purposes. Hence the form factors for various lead-acid batteries are common.